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        'use strict';
        function area_of_circle(r, pi) {
            if (arguments.length === 1) pi = 3.14;    
            return r*r*pi;
        }
        // 测试:
        console.log(area_of_circle(2));
        console.log(area_of_circle(2, 3.1416));
    </script>
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